1. Field of the Invention
This invention relates to a signal processing device and similar of a radar transceiver which receives reflected signals of transmission signals subjected to frequency modulation such that the frequency rises and falls and which generates beat signals having the frequency corresponding to the frequency difference between the transmission signals and reception signals, and in particular relates to a signal processing device and similar which uses beat signals generated when the frequency change rates of transmission signals are different to detect the relative distance or relative velocity of a target object.
2. Description of the Related Art
In recent years, vehicles have been equipped with radar devices employing FM-CW (Frequency Modulated-Continuous Wave) designs, as obstruction detection means in the vehicle. FM-CW radar devices transmit and receive radar signals subjected to frequency modulation such that the frequency rises and falls, and detect the relative distances and relative velocities of target objects.
FIG. 1A to FIG. 1D explain the principle of detection of the relative distance and relative velocity by an FM-CW radar device. In FIG. 1A to FIG. 1D, the horizontal axis indicates time and the vertical axis indicates frequency. FIG. 1A shows the frequency changes in the reception signals and in the transmission signals transmitted by the FM-CW radar device. The frequency of transmission signals rises and falls linearly (ramps up and down), with a central frequency f0 and frequency modulation width ΔF, according to the frequency modulation signal with a sawtooth waveform and frequency fm, as indicated by the solid line. On the other hand, reception signals which are reflected by the target object and return have a temporal delay of ΔT due to the relative distance, and a frequency shift by a Doppler shift amount γ according to the relative velocity, as indicated by the dashed line. As a result, in the transmitted and received signals there occur a frequency difference a in the transmission signal's frequency ramp up interval (up interval), and a frequency difference β in the frequency ramp down interval (down interval). By mixing transmitted and received signals, the radar device generates beat signals having frequencies corresponding to the frequency differences between the transmitted and received signals, and the beat signal frequencies (beat frequencies) are analyzed to detect the frequency differences. Then, the following equations are used to detect the relative distance R and relative velocity V of the target object. Here C is the speed of light.R=C·(α+β)/(8·ΔF·fm)V=C·(β−α)/(4·f0)
Here, when the positive or negative relative velocity of the target object increases in the state of FIG. 1A, the Doppler frequency shift widths of the reception signals increase, and frequency differences between the transmission and reception signal decrease in the up and in the down intervals. And as shown in FIG. 1B, when the frequency shift is γ1 or γ2, beat frequency detection is no longer possible. When in the state of FIG. 1A the relative distance of the target object increases, the time delay in the reception signal (ΔT1) increases as shown in FIG. 1C, and as a result the beat frequency becomes as large as α1 and β1, and in particular the beat frequency β1 in down intervals, becomes large. And, when the beat frequency exceeds the Nyquist frequency of the reference clock in the signal processing device, beat frequency detection is no longer possible. When the relative distance increases further, the delay time ΔT2 exceeds ½ the wavelength of the sawtooth wave as shown in FIG. 1D, the transmission and reception signals no longer overlap in the up intervals or down intervals, and beat frequency detection is no longer possible. Thus due to the principle of the FM-CW method, there are limits to the relative distances and relative velocities which can be detected.
FIG. 2 shows the range of relative distances and relative velocities which can be detected in the above method. The horizontal axis indicates relative distance and the vertical axis indicates relative velocity. The boundary lines b1, b2 correspond to the states indicated in FIG. 1B and FIG. 1C respectively; the boundary point b3 corresponds to the state indicated in FIG. 1D. Hence within the range in which detection is possible (detectable range), bounded by the boundary lines b1, b2 and the boundary point b3, the relative distance and relative velocity are detected.
A radar device for vehicles is required to detect rapidly approaching target objects (objects with a large negative relative velocity) at short distances, due to the need for collision avoidance control and collision response control. That is, it is required that the relative distance and relative velocity for a target object be detected in a region a1 outside the detectable range in FIG. 2. Or, due to the need to control following travel in congested traffic, it is required that a target object be detected which is rapidly moving away (large positive relative velocity) at a short distance. That is, it is required that the relative distance and relative velocity of a target object be detected in the region a2, outside the detectable range in FIG. 2.
To address these requirements, a method has been proposed in which transmission signals are subjected to frequency modulation with a large frequency change rate and frequency modulation with a small frequency change rate, and by combining the beat signals obtained from both these cases, the relative distance and relative velocity are detected; one example is described in Japanese Patent Application Laid-open No. 2004-151022.
In this method, as shown in FIG. 3A, a transmission interval T1, in which transmission signals are transmitted having a large transmission signal frequency change rate (that is, a large absolute value of the rate of change of the frequency, represented by the inclination of the sawtooth wave), and a transmission interval T2, in which transmission signals are transmitted having a small transmission signal frequency change rate (that is, a small absolute value of the rate of change of the frequency, represented by the inclination of the sawtooth wave), are provided. The beat frequencies of beat signals generated in the up intervals of each of these transmission intervals, or the beat frequencies of the beat signals generated in the down intervals, are combined, and the following equations (1) through (4) are used to calculate the relative distance R and relative velocity V.
[When combining beat frequencies in up intervals]R=(2·α2−α1)/(2·4·ΔF2·fm2/C)  (1)V={ΔF1·fm1·α2/(ΔF2·fm2−ΔF1·fm1)−α1}/(4·f0/C)  (2)[When combining beat frequencies in down intervals]R=(2·β2−β1)/(2·4·ΔF2·fm2/C)  (3)V={ΔF1·fm1·β2/(ΔF2·fm2−ΔF1·fm1)−β1}/(4·f0/C)  (4)
In equations (1) through (4), for the transmission interval T1 the transmission signal frequency change width is ΔF1, the up interval beat frequency is α1, the down interval beat frequency is β1, and the sawtooth wave frequency is fm1; for the transmission interval T2 the transmission signal frequency change width is ΔF2, the up interval beat frequency is α2, the down interval beat frequency is β2, and the sawtooth wave frequency is fm2. The central frequency of the transmission signals for both intervals is f0, and the speed of light is C.
The range in which the relative distance and relative velocity can be detected using this method is shown in FIG. 3B. That is, the relative distance and relative velocity are detected in the region a2 in which detection is possible using the equations (1) and (2). And, the relative distance and relative velocity are detected in the region a1 in which detection is possible using the equations (3) and (4).
However, in a radar device for vehicles, when the relative distance and relative velocity are detected within a constant error range over a certain number of consecutive detection cycles, the detection results are regarded as having continuity and are output to a control device of the vehicle. By this means, the accuracy of the detection results is guaranteed.
In the above-described method, the transmission interval T1 in which the frequency change rate is large and the transmission interval T2 in which the frequency change rate is small are processed as a single detection cycle. Hence when for example the number of detection cycles taken to be required for a judgment of continuity is three, in order to obtain a detection result which can be output, the time to execute both the transmission intervals T1 and T2 three times is required. In other words, by using two types of frequency modulation, the detection cycle is lengthened, and to this extent the detection result output is delayed. As a result, there is the problem that the timing of vehicle control is delayed.